June 27, 2013
J-PARC Center

On a delay in suspending the operation of the accelerator complex and a delay
in turning off the ventilation fans at the Hadron Experimental Hallin association with the radioactive material leak accident at the Hadron
Experimental Facility of J-PARC

First, we would like to express our deep apologies for the accident that occurred atthe Hadron Experimental Facility (hereafter HD Facility) of J-PARC on 23 May 2013.
We note that the operation of the accelerator systems at J-PARC was not promptly
suspended in association with this accident. We also note that the ventilation fans in theHadron Experimental Hall (hereafter HD Hall) were operated for a prolonged period oftime. Delays in acknowledging the occurrence of the accident and inadequate
communication caused the operation of the accelerator systems to continue for sometime. Also, an entangled situation at an early stage of responding to the accident led to afailure in promptly issuing instructions for turning off the ventilation fans. We deeply
regret over these serious oversights which caused strong concerns particularly among
the members of local communities, and would like to describe in what follows thesituations that led to the delays in taking these actions.
1. The delay in terminating the second operation of ventilation fans in the HD Hall.
(1) On May 23, we started the second operation of ventilation fans at around 17:30.
When checked at around 18:00 of the 24th, the area monitors at the boundariesof the radiation controlled area of the HD Facility were already indicating normalvalues. They continued to indicate normal values afterwards.
(2) On May 25 during the on-site investigation by officers from the local authorities,
we were inquired over possible safety concerns associated with operation of theventilation fans. Since the dose rates had returned to a normal level, weresponded that there were no safety concerns. (Normal values from the area
monitors were re-confirmed on May 25.)

From then on, during the activities of responding to the accident, it did not cometo our attention to consider terminating the operation of the ventilation fans until
11:26 of May 26, when a facility-wide shut-down procedure was conducted.
(3) Figure 1 shows the data from a gamma-ray area monitor in the HD Hall. It
indicates that most of the release of radioactive material to outside the HD Hall
took place within approximately 3 hours after starting the second operation ofthe ventilation fans at around 17:30.
Figure 2 shows the data from the gamma-ray area monitor, indicating a continual
decrease from then on. This is due to decays of the radioactive material stuck on
the floor surfaces and other objects in the HD Hall.
Figure 3 shows the locations of monitoring posts and monitoring stations at theNuclear Science Research Institute and the Nuclear Fuel Cycle Engineering
Laboratories (NCL). Figure 4 shows the measured values of radiation doserates.
Three monitoring locations of the NCL’s monitoring system observed temporary
rises in the dose rate at around 15:00 and around 17:30 on May 23, in
coincidence with the start-up of the operation of the ventilation fans of the HD
Hall. The subsequent data shows the values within the range of normal variation.
Other monitoring posts and stations show the data to be within the range ofnormal variation throughout the same period, irrespective of the operation of theventilation fans. From these observations, we consider that there was no releaseof radioactive material to outside the HD Facility nor were there any influences
caused by the continuous operation of the ventilation fans after 21:00 on May23.
2. Delay in suspending the operation of accelerator complex【50 GeV synchrotron (hereafter MR)】・ Beam operation in support of users’ experiments of the MR was stopped ataround 16:15 on May 23, along with the closure of the HD Facility.
・ Tuning operation of MR was conducted from 14:47 to 22:46 on the next day,May 24.

・ In this tuning operation, a proton beam was injected into the MR every several toa few over ten minutes. No beam acceleration, however, was made, and thebeam power was below 1/10 of that of the normal operation in support of users’
experiments at the HD hall.
・ In the tuning operation, the beams from the MR were dumped onto thedesignated beam dump in the accelerator tunnel. No beam extraction was done
toward the HD facility.
・ At 22:46, when the tuning was completed, we stopped the tuning operation ofthe MR.
【Linear Accelerator (LINAC) and 3 GeV Synchrotron (RCS)】・ At 22:15 on May 24 the an on-site response headquarters determined that theincident on the previous day (May 23) is classified under the statutory provision
for reporting as an accident, and after reporting the case to the relevant
authorities and organizations, we stopped the beam operation of the LINAC andRCS at 0:46 on May 25, as instructed by the Director of J-PARC Center.
【 Neutrino Experimental Facility (NU Facility) and Materials and Life ScienceExperimental Facility (MLF)】
・ No experiments were being carried out in the NU Facility on May 23 and 24.
Only equipment maintenance work, adjustment of devices and related activities
were being conducted.
・ Experiment(s) with using neutrons and muons were being carried out in the MLF
on May 23 and 24. Execution of all the experiments was terminated at 0:46 on
May 25, as instructed by the Director of J-PARC Center.
Reflecting on inadequate responses described above, we commit ourselves to makeevery effort in investigating the causes of this accident and in creating measures toprevent recurrence of similar accidents with inputs and advices from the External Expert
Panel.